In a WiMAX (Worldwide Interoperability for Microwave Access) radio communication system, the transmission and reception of data are carried out in subframe units.
In a WiMAX radio communication system, OFDMA (Orthogonal Frequency Division Multiple Access) is adopted for multiple access, and TDD (Time Division Duplex) is adopted for duplex.
OFDMA is a method of dividing the frequency direction by subcarriers and dividing the time direction by time slots and assigning subcarriers and time slots in which the subcarriers can be used to uplink and downlink data.
TDD is a method of using the same frequency in uplink and downlink and switching uplink subframes and downlink subframes on a time axis.
As a result, in a WiMAX radio communication system, frame synchronization must be established between base stations to synchronize the timing of the transmission of downlink subframes. In addition, a base stations must also establish time synchronization with UTC (Coordinated Universal Time) with an accuracy of 10 μsec (optionally, an accuracy of 1 μsec).
For this purpose, a base station normally incorporates a GPS (Global Positioning System) module and, by controlling the timing of the transmission of downlink subframes based on UTC time information received from a GPS satellite, and thereby base station realizes the time synchronization with UTC and inter-base station frame synchronization.